1. Field of the Invention
The invention relates to a process for calibrating measurement sections for measuring the interference field strength of electrical devices, for example for determining calibration factors for measuring the interference field strength by means of the frequency spectrum, in particular up to 1 GHz, where fields are generated by means of a comb generator in a known reference measurement section, for example a standard measurement section or an open air test site, and in a measurement section to be measured, with one and the same calibration device (radio interference emitter), in particular with the same operating parameters, these fields are each measured in the same way and the calibration factors are calculated from the comparison of the measured values as a function of the frequency. Finally, the invention also relates to a process for determining the calibration factors of a measurement section or a process for simulating the interference emissions of test object or actual electrical devices.
The invention furthermore relates to a comb generator with a possibly battery-operated oscillator to power an antenna. Furthermore, the invention relates to a calibrating device for measuring devices or measurement sections for interference field strength measurements, comprising a battery-operated comb generator, possibly of the type of the invention, which comb generator is employed for powering the antenna and which is particularly free of metallic conductors. Furthermore, the invention relates to a radio interference simulator, also with a possibly battery-operated comb generator.
2. Discussion of the Background of the Invention and Material Information
Processes for calibrating measurement sections for interference field strength measurements of electrical devices are known to the extent that in a known reference measurement section the measuring data are obtained from a single transmitting antenna and appropriate measurements are taken in the measurement section to be calibrated at the same radio interference transmitter and the appropriate calibration factors are obtained from a comparison of the two measurements. Such a process is known, for example, from e&i-Elektrotechnik & Informationstechnik (e&i Electrical Technology & Information Technology), Vol. 106 (1989), No. 4, pp. 134 to 145, or IEEE 1989 National Symposium on Electromagnetic Compatibility, May 23-25, p. 391, column 1. A similar process is known from 1986 IEEE International Symposium on Electromagnetic Compatibility, Sep. 16-18, 1986, pp. 362 to 365.
A direction-finding antenna system, using a dipole and a coil antenna is known from German Patent Document DE-A-2 602 476 which, however, cannot be used for calibration purposes.
The triggering of at least two antennas having different transmitting characteristics for improving the response properties of a radar installation is known from Patent Abstracts of Japan, Vol. 9, No. 210 (P-383) 1938, Aug. 28, 1985; however, such a radar installation cannot be used for calibrating measurements of the type mentioned.
Finally, the use of a bipolar transistor operated in a controlled avalanche breakthrough mode, the frequency-stable signal of which obtained during breakthrough extends up to 1 GHz, for providing a pulse generator is known from Q.S.T. amateur radio, Vol. 56, No. 5, May 1972, pp. 15, 16. The use of such transistors for comb generators or the use of such pulse generators for powering antennas for calibrating purposes, however, cannot be found in this reference.
Electrical and electronic devices generate electromagnetic fields. This is a result of the physical fact that every electrical current which can be changed over time results in the emission of electromagnetic waves.
In the case of a radio transmitter, these waves are generated on purpose and are desirable (radio waves). Electrical currents which can be changed over time are also employed or generated in electrical or electronic devices, for example in computers, household devices or the like. Although these currents are purposely generated, the wave emission or field generation connected with the currents is not desirable, since the function of other electrical or electronic systems can be interfered with because of them. In particular, undesirable effects of this type have to be prevented where electronic systems which are relevant in connection with safety could be interfered with, for example the control electronics in aircraft or motor vehicles, medical-technical devices, etc. For this reason the undesirable wave emission by electronic devices must be reduced to less than a maximally permissible value by technical means, for example shielding of housings, filtering of lines, etc.
An important indicator of the quality of the products of the electrical and electronic industry is their "electromagnetic compatibility". To test the presence of this quality in a product it is necessary to measure not only the active interference capability (emission of electromagnetic fields), but also the interference resistance (against penetration of such fields).
Limit values of the permissible interference emission or interference field strength are determined by the licensing offices (for example the Austrian Postal and Telegraphic Administration) for the area of emissions. The measuring processes for determining the interference field strength generated by a test object are standardized. The measurements can be performed either in the open, i.e. on a so-called "open air test site", or in electromagnetically shielded sheds lined with high-frequency absorbers (absorber sheds). The latter have the advantage of being free from interference by extraneous fields (radio transmitters) as well as being independent of weather. The high-frequency absorbers are required for the reduction of the reflection by the walls of the electromagnetic waves emitted by the test object.
The wave propagation conditions of a measuring site used for interference field strength measurements have an important effect on the measured result. Completely different measured values can be obtained from one and the same test object if two operating sites are selected which are different in regard to the wave propagation conditions. In particular in measuring sheds, undesirable reflections by the absorber walls can distort the measured results in an undue manner. Calibration of the measurement section is the only remedy.
A measuring procedure was standardized (DIN-VDE 0877, Part 2) for calibrating an open field measuring site. There is no precise determination for absorber sheds; the method standardized for the open field is completely insufficient for absorber sheds because completely different environmental conditions prevail. In this type of calibrating measurement sections in accordance with the concept of field damping, the transmission damping between two equally polarized half-wave dipoles erected on the open field measuring site is designated as field damping, if standardized spatial arrangements and measuring processes are observed, such as described in VDE 0877, Part 2 (1985). Field damping measured in the actual field must be compared with the corresponding, theoretically calculated values of an ideal site, where deviations during the interference field strength measurements of test objects on this field are taken into consideration as correction values. Thus, the calculation of the calibration factors on the basis of a comparison of open field measurement site measuring values with theoretical values is performed.
With this method it is assumed that measurement site properties which are not ideal have the same effect on field damping measurements as on field strength measurements of test objects. However, this is particularly not the case, because actual test objects may have completely different geometric dimensions than resonating half-wave dipoles and it is possible that test objects generate a completely different field distribution than the dipole antennas employed for calibration.
Strictly speaking, correction terms determined by field damping measurement therefore most likely do not apply to actual test objects; this fact can lead to impermissibly high measurement errors, in particular in the course of interference field strength measurements in absorber sheds which had been calibrated in accordance with the concept of field damping.
Furthermore, radio interference emitters with dipole-like transmission antennas are known, which include battery-operated comb generators fixedly connected with a linear (dipole-like) transmission antenna. Such devices can be employed for comparison measurements between different measurement sections; in this case, again, only a quite specific field distribution is generated and all results of such comparison measurements again only apply to this single field distribution. A generalization of the measurement results obtained to include arbitrary actual test objects is not possible in absorber sheds measured in this way.
Furthermore, radio interferences with a spherical dipole antenna are known, which include battery-operated comb generators installed in two metallic, electrically insulated hemispheres of a ball, which constitutes a spherical dipole. Such devices can be used in the same way as the previously described radio interference emitters, but again only one particular field distribution is being measured and the measurement results obtained therefore do not have general validity.
Thus all three known types of calibrating devices have the disadvantage that each can only generate a single type of field distribution; however, as a rule actual electrical devices generate a plurality of highly different field images. In order to be able to perform interference field strength measurements at frequencies below approximately 150 MHz in the absorber sheds predominantly employed in today's EMC (electromagnetic compatibility) measuring technology, it is necessary that the measurement section in the shed be calibrated for all actually occurring field distributions. If calibration is performed for only one field distribution and if these calibration factors are used in all testing of electronic devices, the measuring uncertainty can increase far beyond the acceptable value.
A pure interference emitter is furthermore known from German Published, Non-Examined Patent Application DE-OS 3 322 325, which includes a conventional comb spectrum generator, the pulses of which are filtered and amplified and are then supplied to the antenna. With this it is intended to generate interference fields at a considerable distance by means of high transmission output. This known interference emitter is unsuitable for calibrating measurement sections, because its frequency band is limited and the emitted output is not designed for near field measurements.